The ability to create devices such as biosensors, microarrays, and microelectromechanical systems (“MEMS”) requires facile methods to precisely control the devices' surfaces. A variety of patterning techniques can be used to produce desired structures, while various methods have been investigated to control surface chemistries. For instance, microfabrication techniques are routinely applied to create patterned inorganic surfaces having nanometer to micrometer scale resolution. However, traditional approaches have not proven particularly successful in adequately bonding organic and biological materials to the patterned inorganic surfaces.
Several approaches have emerged to extend microfabrication techniques for the creation of patterned surfaces with organic and biological materials. One approach is based on an extension of photolithography, and involves selectively irradiating self-assembled monolayers to create a pattern of freshly exposed surface, which is then reacted with a bifunctional agent and a molecule of interest. Reactions include those between thiols and metal surfaces, or between silanes and oxidized silicon (see Bain, C. D., Whitesides, G. M. Angew. Chem. Int. Ed. Eng/. 1989, 28, 506-512; Whitesides, G. M., Laibinis, P. E. Langm. 1990, 6, 87-96; Sagiv, J. J. Am. Chem. Soc. 102, 1980, 92-98; Brzoska, J. B., Azouz, I. B.; Rondelez, F. Langm. 1994, 10, 4367-4373; Allara, D. L., Parikh, A. N., Rondelez, F. Langm. 1995, 11, 2357-2360).
In such methods, a first functional group of the bifunctional agent attaches the agent to the freshly exposed surface, and the second functional group subsequently reacts with the molecule of interest, thereby conjugating it to the surface. Although variations exist, lithography is typically employed to create the spatial template upon which the subsequent conjugation occurs. This first approach has several drawbacks: the required photo-sensitive reagents can be expensive and hazardous to use; additionally, cumbersome steps are required in order to prepare the surface. Furthermore, conventional photolithographic operations require “line-of-sight” and cannot be readily employed on internal surfaces (such as in an enclosed microfluidic system). Alternatively, if the lithographic patterning and subsequent biological functionalization are carried out before the microfluidic device is covered to form a closed fluidic environment, the biofunctionality internal to the microfluidic system cannot be readily reprogrammed. Finally, since many biospecies are labile, i.e., sensitive and delicate with respect to their environmental conditions, fabrication processes required to close the microfluidic system may degrade the biospecies.
A second approach for creating patterned surfaces with organic and biological materials is microcontact printing (“μCP”). In μCP, a soft stamp (typically made of poly-dimethylsiloxane) is created with a preselected pattern. After “inking” the stamp with a solution containing the material to be deposited, the stamp is pressed onto the surface to transfer the pattern. Drawbacks to the microcontact printing approach involve difficulties in stamping with high spatial resolution. Furthermore, the need for direct contact to the surface entails the drawbacks described above for applications to enclosed microfluidic systems (Vaeth, K. M., Jackman, R. J., Black, A. J. Whitesides, G. M., Jensen, K. F., Langmuir 2000, 16, 8495-8500).
Another approach to patterning biomolecules on surfaces is known as “dip-pen” nanolithography.” In this process, scanning probe microscopy (similar to atomic force microscopy) is used to “write” species onto a surface with high lateral resolution. For biomolecular species this is accomplished by transport from the writing tip through a water meniscus to the substrate. While the lateral spatial resolution of this patterning method can be very high (30 nm), patterns must be written in serial fashion, entailing throughput limitations similar to those associated with other direct-write approaches such as electron and ion beam lithographies. In addition, dip-pen nanolithography entails the drawbacks described above for applications to enclosed microfluidic systems (Piner, R. D., Zhu, J. Z., Xu, F., Hong, S., Mirkin, C. A., Science 29 Jan. 1999, 283, 661-663; Jong, S., Mirkin, C. A., Science 9 Jun. 2000, 288, 1808-1811; Lyuksyutov, S. F. et. Al., Nature Materials July 2003, 2, 468-474).
Electrophoretic deposition has also been used to assemble colloidal particles and proteins onto electrode surfaces. This approach has been extended to exploit an electric field to direct the spatially selective deposition of CdTe nanocrystals (Gao, M, et al, Langmuir, 18, 4098-4102 (2002)). In this method, a surface with patterned electrodes is first fabricated, and then a combination of an applied voltage and layer-by-layer assembly is used to generate multilayers with spatial resolution in lateral directions. The drawbacks to this assembly approach are that voltages must be maintained to retain the initial layer of nanocrystals, which may not be held to the surface by strong chemical bonds or insolubility. Again, it is not clear from these documents whether these layer-by-layer approaches can be extended to enclosed microfluidic channels.